I believe that comment is more or less based on the shuttle landing and/or the soyuz capsules (eg parachutes) - but as far as I can tell, SpaceX is the only one currently capable (and using) propulsive landing on earth - so essentially they've already leapfrogged the grey zone to a large extent.
Makes me wonder whether testing a propulsive landing on an airborne drone barge is feasible? A blimp with a well-insulated large pad on the top - though I'm not sure how you'd effectively anchor it in the air - probably a active propulsion system as well.
Makes me wonder whether testing a propulsive landing on an airborne drone barge is feasible? A blimp with a well-insulated large pad on the top - though I'm not sure how you'd effectively anchor it in the air - probably a active propulsion system as well.